This invention relates generally to analog and mixed signal (analog and digital) integrated circuits, and more particularly to bandgap voltage references used in analog and mixed signal integrated circuits.
Reference voltages are required for a variety of purposes. For example, reference voltages are used in analog to digital (A/D) converters, and in the regulation of d.c. power supplies. A problem inherent with voltage references is that their output voltages tend to be temperature-dependent. This is because active devices, such as transistors, of the circuitry have operating characteristics (e.g. base current and V.sub.BE) which vary according to temperature. It is, of course, desirable to minimize the temperature-dependency of the voltage reference circuitry to provide a stable reference voltage.
It is known in the art that a "bandgap" voltage reference is quite stable over a range of temperatures. As it is well known to those skilled in the art, the bandgap of a semiconductor is the energy difference between the bottom of the conduction band and the top of the valance band for the semiconductor. Since the bandgap voltage of silicon is 1.2 eV, a bandgap voltage reference of +1.2 volts d.c. is selected as a stable reference voltage for silicon-based transistor and integrated circuit technologies. Bandgap voltage references of the prior art generally operate by summing the base-emitter voltage V.sub.BE, of a bipolar transistor with a proportional-to-absolute-temperature (PTAT) voltage V.sub.PTAT, which is typically developed across a PTAT voltage drop resistor.
In FIG. 1a, a prior art bandgap voltage reference circuit 10 is illustrated. The voltage reference circuit 10 and variants thereof are commonly known as "Widlar" bandgap circuits. A Widlar bandgap circuit 10 includes a first transistor 12, a second transistor 14, and a third transistor 16. The transistors 12, 14, and 16 are all NPN bipolar transistors. A bandgap reference voltage V.sub.REF is developed at a node 18 and is connected to a load 20, such as the aforementioned A/D converter, d.c. power supply, etc. The collector of transistor 12 is coupled to V.sub.REF by a resistor 22, and the emitter of transistor 12 is coupled to ground. The base of transistor 12 is coupled to its collector to cause the transistors 12 and 14 to be current mirrors. A resistor 24 couples the collector of transistor 14 to V.sub.REF, and the base of transistor 14 is coupled to the base of transistor 12 by line 24. The emitter of transistor 14 is coupled to ground by a resistor 26. Transistor 16, which serves as an error-feedback device, has its collector coupled to V.sub.REF, and its emitter coupled to ground. The base of transistor 16 is coupled to the collector of transistor 14. The Widlar bandgap circuit 10 is powered by a current source 28 coupled to a power supply V.sub.cc. The size of transistor 14 is made larger than the size of transistor 12 to compensate for the voltage drop across resistor 26. In bipolar technology, a transistor is made larger than another transistor by having a relatively larger emitter. In this instance, the emitter of transistor 14 may be, for example, four, eight, or ten times larger than the emitter of transistor 12.
The Widlar bandgap circuit 10 operates as follows. The circuit 10 creates a bandgap voltage reference V.sub.REF =V.sub.PTAT +V.sub.BE due to the current I.sub.PTAT flowing though resistor 22 and transistor 12. The current source 28 attempts to keep a constant current flowing into a node 30. The error transistor 16 takes a certain amount of the current from node 30 and shorts it to ground. The remaining current flows through transistors 12 and 14. When in regulation, the base of transistor 16 is close to the voltage at the base of transistor 14. This allows the transistor 16 to shunt an amount of current such that the total combined current between transistors 12 and 14 is proportional to the absolute temperature (T.sub.K). As T.sub.K varies, the current through transistors 12 and 14 varies linearly relative thereto, maintaining the voltage V.sub.REF at the desired 1.2 volts d.c. If the voltage on V.sub.REF attempts to rise, the current through transistor 16 increases, decreasing the amount of current flowing through transistors 12 and 14 and therefore pulling down on V.sub.REF. If V.sub.REF attempts to drop, the current flowing through transistor 16 decreases, increasing the amount of current flowing through transistors 12 and 14, thereby intending to increase V.sub.REF to its regulated 1.2 volts D/C. Therefore, the transistor 16 controls the total current flowing through transistors 12 and 14 to maintain the level of V.sub.REF, i.e. to cause I.sub.PTAT to flow through resistor 22 and transistor 12.
The Widlar bandgap circuit 10 suffers from a built-in error. This is due to the fact that the base current for both transistors 12 and 14 flows through the resistor 22, causing a voltage drop. While the base current is relatively small, it still can produce an error of approximately 200 parts per million per degree Celsius (ppm/.degree. C.) in the regulated voltage V.sub.REF. While this level of accuracy is satisfactory for certain applications, other high-precision applications, such as a high-precision A/D converter, requires a reference voltage with higher levels of accuracy.
A prior art bandgap voltage reference circuit 32 known as a "Brokaw Cell" is shown in FIG. 1B. The circuit 32 includes a first transistor 34, a second transistor 36, and an error amplifier 38. The collector of transistor 34 is coupled to V.sub.cc by a resistor 40, and its emitter is coupled to ground by a resistor 42. The base of transistor 36 is coupled to the base of transistor 34. The collector of transistor 36 is coupled to V.sub.cc by a resistor 44 and the emitter of transistor 36 is coupled to ground by the series connection of a resistor 46 with the aforementioned resistor 42. The inputs of the error amplifier 38 are coupled to the collectors of transistors 34 and 36 and the output of error amplifier 38 develops the reference voltage V.sub.REF at a node 48. A load 50 is coupled between the output node 48 and ground. The output of the error amplifier 38 is also fed-back to the bases of transistors 34 and 36 by a line 52, i.e. the output of the error amplifier provides the base currents for transistors 34 and 36.
With the Brokaw cell 32, the transistor 36 is again larger than the transistor 34 to allow an equalization of the currents flowing through those two transistors. In operation, and if resistors 40 and 44 are of the same value, the error amplifier 38 attempts to regulate the current so that equal current flows through transistors 34 and 36. The current through these two transistors is proportional to the absolute temperature in Kelvin (T.sub.K). The voltage drop across resistor 42, plus the voltage drop V.sub.BE of transistor 34 is used to generate the bandgap voltage V.sub.REF. The Brokaw cell 32 does not suffer from the aforementioned base current error problem of the Widlar bandgap circuit, because the base currents are supplied by the error amplifier 38, not through a PTAT voltage drop resistor.
The Brokaw cell does, however, have a significant drawback in that it requires considerable "head room," (i.e. the voltage differential between V.sub.cc and V.sub.REF) for proper operation. Since the Brokaw cell 32 typically requires at least 1 volt of head room, this limits the Brokaw cell technology to applications wherein V.sub.cc is greater than about 2.2 volts. This means that it is difficult to "bootstrap" a Brokaw Cell by powering the cell with its own output V.sub.REF. It is desirable to have a bootstrapped bandgap voltage reference since it is more stable than a bandgap voltage reference operating from V.sub.cc or some other voltage source. This is because, almost by definition, the bandgap voltage reference is the most temperature-stable voltage source available to power the circuit. While it is possible to provide a bootstrapped Brokaw Cell, such a cell is quite complex in design, and requires a considerable amount of valuable real estate on the integrated circuit. The aforementioned Widlar bandgap circuit is a bootstrap circuit, but that advantage is overshadowed by the error caused by the base current flowing through the PTAT voltage drop resistor.